Nearly full-dense and fine-grained AZO:Y ceramics sintered from the corresponding nanoparticles
© Yang et al.; licensee Springer. 2012
Received: 20 June 2012
Accepted: 22 August 2012
Published: 29 August 2012
Aluminum-doped zinc oxide ceramics with yttria doping (AZO:Y) ranging from 0 to 0.2 wt.% were fabricated by pressureless sintering yttria-modified nanoparticles in air at 1,300°C. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, a physical property measurement system, and a densimeter were employed to characterize the precursor nanoparticles and the sintered AZO ceramics. It was shown that a small amount of yttria doping can remarkably retard the growth of the as-received precursor nanoparticles, further improve the microstructure, refine the grain size, and enhance the density for the sintered ceramic. Increasing the yttria doping to 0.2 wt.%, the AZO:Y nanoparticles synthetized by a coprecipitation process have a nearly sphere-shaped morphology and a mean particle diameter of 15.1 nm. Using the same amount of yttria, a fully dense AZO ceramic (99.98% of theoretical density) with a grain size of 2.2 μm and a bulk resistivity of 4.6 × 10−3 Ω·cm can be achieved. This kind of AZO:Y ceramic has a potential to be used as a high-quality sputtering target to deposit ZnO-based transparent conductive films with better optical and electrical properties.
KeywordsAZO:Y Ceramic Nanoparticles Coprecipitation Sintering Finer grain Nearly full density 61 61.66.Fn 61.72.uj
Transparent conductive oxides (TCO) as transparent electrodes have been widely used in thin-film solar cells and flat panel display devices[1, 2]. The commonly applied TCO materials are In2O3:Sn (ITO), SnO2:F (FTO), and ZnO:Al (AZO)[1, 2]. AZO has attracted much interest as a potential substitute for ITO due to the abundance of its constituent elements in nature, relatively low deposition temperature, and stability in hydrogen plasma[2, 3].
The magnetron-sputtering ceramic target is one of the most widely used methods for AZO film deposition. In the sputtering system, the target plays a major role in achieving high-quality films[4–7]. Generally, the target for sputtering TCO films should have a high density, finer grain size, and better conductance[7–11], which will be helpful for avoiding the formation of nodules to prolong the target lifetime[7, 8], increasing the deposition rate and film uniformity and meeting the requirement of direct current (DC) sputtering. The attempts to enhance the density of the AZO ceramic target become a crucial issue for both researchers and target manufacturers[7, 8, 12–14]. Sun et al. fabricated an ultrahigh-density AZO sintered body (>99.7% theoretical density) after pressureless sintering at 1,400°C by adjusting the mass fraction of polyacrylic acid when slip casting a mixture slurry of commercial ZnO and 2 wt.% Al2O3 powders. Hwang et al. found that the preliminary heat treatment under external pressure increased the density and uniformity after a final sintering. The maximum density value of 2 wt.% Al-doped ZnO sintered at 1,350°C was about 5.52 g/cm3 (approximately 98.9% of the theoretical density). Recently, Zhang et al. used a two-step sintering process to obtain the AZO ceramic with a relative density of more than 99% by sintering the 30-nm sol–gel-synthesized AZO nanoparticles at the second-step sintering temperature of 1,000°C for 12 h. To our best knowledge, the nearly full-dense (namely, exceeding 99.9% of the theoretical density) AZO ceramic has rarely been reported and still kept a challenge as before, especially by a simple and low-cost pressureless sintering at a relatively low temperature.
Few studies have also shown that a small amount of a rare earth element such as yttrium introduced into a ZnO matrix can obviously improve the properties of both ZnO films and the corresponding ceramic sputtering targets[15–18]. For example, Han et al. have utilized an electrochemical deposition method to obtain a 3.7 at% yttrium-doping ZnO film with a resistivity of as low as 6.3 × 10−5 Ω cm after post-deposition annealing in nitrogen at 300°C. GfE Co. (Nuremberg, Germany) has produced a novel aluminum-doped zinc oxide ceramic with yttria doping (AZO:Y) target containing a small amount of Y2O3 besides Al2O3, which can be stably sputtered by pulsed DC sputtering technology due to the higher conductivity[17, 18]. Using this kind of target, Tsai et al. found that the thin AZO:Y film deposited at 300°C had the lowest resistivity of 3.6 × 10−4 Ω cm, the highest mobility of 30.7 cm2 V−1·s−1, and the highest carrier concentration of 5.6 × 1020 cm−3.
However, the above mentioned research results mainly focused on the properties of AZO:Y films; the detailed investigation on the influence of Y doping on the mircrostructure and densification of the AZO ceramic target itself is lacking. In this work, we attempted to fabricate highly dense AZO:Y ceramics by pressureless sintering by a coprecipitation process using Y and Al co-doped ZnO nanoparticles as raw materials, and the microstructure and densification of AZO:Y ceramic were investigated.
Synthesis of AZO:Y nanoparticles
Y-doped AZO (AZO:Y) nanoparticles were synthesized by a coprecipitation process, using an AR grade of zinc nitrate, aluminum nitrate, yttrium nitrate, and ammonium acid carbonate as starting materials (all purchased from Sinopharm Group Co. Ltd., Shanghai, China). A 1 M distilled water solution of Zn(NO3)2·6H2O, Al(NO3)3·9H2O, and Y(NO3)2·6H2O, whose amounts were determined by Al2O3/[ZnO + Al2O3] = 2 wt.% and Y2O3/[ZnO + Al2O3] = 0, 0.1, 0.15, and 0.2 wt.% (the corresponding samples being named AZO:Y0, AZO:Y0.1, AZO:Y0.15, and AZO:Y0.2), respectively, were added to 2 M NH4HCO3 solution drop by drop at a constant temperature of 30°C with stirring to produce a mass white precipitate. After aging for 24 h, the precipitate was filtrated and washed several times, followed by drying for 12 h in an oven at 100°C. Then, the precipitate was calcined at 600°C for 2 h to form AZO:Y nanoparticles.
Sintering of AZO:Y ceramics
The as-received AZO:Y nanoparticles were first granulated by spray drying to form larger sphere aggregations with a diameter of approximately 10 μm and then were pressed by uniaxial pressing (50 MPa, 3 min) in a stainless steel die with a diameter of 8 cm. The green bodies were subsequently pressed by cold isostatic pressing (250 MPa, 5 min) and sintered in air for 8 h at 1,300°C in an electric furnace. In order to clearly observe the microstructure and conveniently measure the conductivity, the sintered specimens were ground and polished with a 1-μm corundum slurry and then thermally etched at 900°C for 20 min.
The phases of the AZO:Y nanoparticles and sintered specimens were identified by X-ray diffraction analysis (XRD, D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany) with CuKα radiation (λ = 1.5406 Å) operated at 40 kV and 40 mA and a scanning step of 0.02°/s. The morphology, microstructure, and composition analyses of the AZO:Y nanoparticles and the sintered bodies were performed using a scanning electron microscopy(SEM)/energy-dispersive X-ray analysis (EDAX) system (S-4800, Hitachi Ltd., Tokyo, Japan). The average particle/grain size of the as-calcined nanoparticle or sintered ceramic specimen was estimated from a minimum of 100 particles/grains obtained from the SEM images by the linear intercept method proposed by Mendelson. The densities of the sintered specimens were determined by Archimedes’s method with a densitometer (MH-600, MatsuHaku Electronic Co., Ltd., Taichung, Taiwan). The bulk resistivities were measured by a physical property measurement system (PPMS; Model-9, Quantum Design Inc., San Diego, CA, USA) at room temperature.
Results and discussion
Bulk resistivity (Ω·cm)
2.1 × 10−3
3.3 × 10−3
4.3 × 10−3
4.6 × 10−3
The main focus of this study was to improve the density and microstructure of the AZO ceramic target by introducing a yttria dopant with a low-cost pressureless sintering process. SEM, EDS, XRD, PPMS, and a densimeter were employed to characterize the precursor nanoparticles and the sintered AZO ceramics. Increasing the yttria doping to 0.2 wt.%, the AZO:Y nanoparticle synthesized by a coprecipitation process has a nearly sphere-shaped morphology and a mean particle diameter of 15.1 nm. With the same amount of yttria, a fully dense AZO ceramic (99.98% of TD) with a grain size of 2.2 μm and a bulk resistivity of 4.6 × 10−3 Ω·cm can be achieved. This kind of AZO:Y ceramic has a potential to be used as a high-quality sputtering target to deposit ZnO-based transparent conductive films with higher optical and electrical properties.
physical property measurement system
The authors gratefully acknowledge the supports from the National Science Foundation of China (20975107, 60806032), the ‘Hundred Talents Program’ of the Chinese Academy of Sciences, the Zhejiang Natural Science Foundation (Y4100169, Y4110463), and the Ningbo Innovative Research Team Program.
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